1. Field of the Invention
This invention relates to ground fault interrupter apparatus of the differential transformer type.
2. Description of the Prior Art
The basic condition for which known types of ground fault interrupters of the differential transformer type are intended to be effective is that in which there is a ground fault between the hot or line conductor and ground. A ground fault between the neutral conductor of a multi-wire A.C. system on the load side of the differential transformer and ground does not itself present a safety hazard of the type for which protection is required. Such a grounded neutral condition has the effect, however, of rendering the apparatus inoperative, particularly in the absence of a load on the circuit. When a leakage current to ground occurs on the load side of a neutral conductor that conductor is effectively shorted since it is intentionally grounded on the input side. By transformer action, the other primary winding, the hot or line conductor, will then have essentially no potential difference across it and the device would be incapable of generating sufficient power to produce a sensing current on the sensing winding sufficient to open the line circuit in the event of a ground fault on that line. In the event of a ground on the line conductor, there is a path back to the source through the grounded neutral conductor, reducing sensitivity of the unit.
Means have been proposed to avoid the adverse effects of the grounded neutral condition. For example, copending application Ser. No. 158,337, filed June 30, 1971, now refiled as continuation application Ser. No. 368,915, June 11, 1973, by K. R. Coley, J. J. Misencik and J. R. Reeves and assigned to the assignee of the present invention, (both of which applications are now abandoned) relates to a grounded neutral detection scheme using capacitive means connected between the neutral conductor and the line conductor. Upon occurrence of a grounded neutral, the capacitive means charges resulting in an unbalanced condition appearing on the neutral and line conductors which can be sufficient to cause tripping. This effect may be used with an additional magnetic core on the input side of the neutral conductor in order to limit current therein and permit use of a smaller capacitor. Alternatively, a magnetic core around the neutral conductor on the input side can be used to provide an effective impedance that avoids the problems of the grounded neutral condition even in the absence of the above mentioned capacitive means.
The foregoing techniques have considerable capability and effectiveness of performance. At the time of that invention, it was considered attractive to employ the disclosed means as providing an improvement in size and cost over other then known techniques such as that of U.S. Pat. No. 3,473,091, discussed below. However, development of the invention of the copending application left remaining a desire to minimize further the size and cost of the elements required for protecting against the grounded neutral condition.
The approach of the above-mentioned U.S. Pat. No. 3,473,091 is to place an impedance in the neutral conductor to keep substantial potential across it even in the event of a low impedance short to ground. In preferred forms shown in that patent, an auxiliary transformer is used whose primary is the line conductor wound in the same manner as in the differential transformer and whose secondary winding is the neutral conductor counterwound to produce reverse polarity from the neutral conductor of the differential transformer. This arrangement of reverse polarity transformer windings is said to make it appear that a high impedance exists in the neutral conductor regardless of any shorted condition on the load side. Alternatively, the primary conductor of the output transformer may be a winding connected between the line and neutral conductors, but with similar effect. A further example of prior art using a transformer induced voltage in the neutral conductor to cause a current imbalance when the neutral conductor is grounded is U.S. Pat. No. 3,506,906.
The use of an auxiliary transformer that is intended to provide continuous transformer action to present a high impedance in the neutral conductor requires an undesirable addition of bulk and corresponding cost that is very important because of the interest in providing ground fault interrupter apparatus in compact form in order to permit their use in association with conventional circuit breakers.
U.S. Pat. No. 3,611,035 discloses a way to protect against the grounded neutral condition in which a high frequency tickler voltage is induced into the neutral conductor through an output transformer. The disclosed system requires a considerable number of components including a semiconductor switch that presents temperature sensitive characteristics and a susceptibility to voltage surges. Thus, while output transformer core size reduction over previous proposals may be achievable, the cost, size and reliability of the ancillary components is considered undesirable.
It was in an effort to provide protection against the grounded neutral condition while minimizing the size and cost of all the elements required, and while maintaining high reliability, that the present invention came about.